The invention relates to pyroelectric targets for use in pyroelectric vidicon tubes, pyroelectric point detectors, solid state pyroelectric imagers, and other pyroelectric devices. The invention also relates to a method of manufacturing pyroelectric targets and to pyroelectric vidicon tubes having such targets.
In the manufacture of a pyroelectric target from a pyroelectric material (i.e. a material which exhibits spontaneous polarization), the material must be cut to produce two flat, parallel faces. The performance of such a pyroelectric target as an infrared vidicon target depends, at least in part, on the figure of merit, M, of the target. The figure of merit, M, is given by the expression ##EQU1## where p is the component of the pyroelectric coefficient of the material in a direction perpendicular to the faces of the target, .epsilon. is the dielectric constant (permittivity) of the material in a direction perpendicular to the faces of the target, and c is the specific heat of the material per unit volume. The pyroelectric coefficient is the rate of change of the spontaneous polarization of the material with temperature.
In selecting pyroelectric materials for use as vidicon targets, it is desirable to choose a material having as high a figure of merit as possible in order to maximize the signal produced from the detected infrared radiation.
For infrared point detectors, the maximum responsivity bandwidth product (and also the open circuit voltage responsivity) are proportional to ##EQU2## where p, c, and .epsilon. are the same quantities discussed above. For the same reason discussed above, it is desirable to choose a pyroelectric material for infrared point detectors having as high a figure of merit as possible.
Having selected a pyroelectric material for use as a target, it is apparent that a high performance can be obtained by maximizing the component of the pyroelectric coefficient which is perpendicular to the faces of the target. The component of the pyroelectric coefficient, p, is thus maximized by cutting the target with its ferroelectric polar axis (pyroelectric axis) perpendicular to the polished faces of the crystal. This pyroelectric axis is defined as being along the direction of spontaneous polarization. For pyroelectric vidicons, such a target and such a method of manufacturing this target is taught in Advances in Image Pickup and Display, Volume 3, "Theory and Performance Characteristics of Pyroelectric Imaging Tubes" by B. Singer, Academic Press, 1977 (edited by B. Kazan). It is similarly known to produce infrared point detectors by electroding the target material normal to the polar (pyroelectric) axis. Such point detectors are known from Principals and Applications of Ferroelectrics and Related Materials, M. E. Lines et al, Clarendon Press, Oxford, 1977 (page 562). Providing a high performance by maximizing the component of the pyroelectric coefficient is also taught in an article entitled "Pyroelectric Detectors and Materials" (Proceedings of IEEE, Volume 66, No. 1, pages 14-26, January 1978), by S. T. Liu et al, in which the pyroelectric material is provided with electroded surfaces normal to the polarization vector. (See, the first page of this article.)
In the known pyroelectric targets and the known methods of manufacturing such targets, although the component of the pyroelectric coefficient normal to the electroded faces of the target is maximized, the figure of merit or the maximum responsivity bandwidth product is limited, in general, by a high permittivity along the pyroelectric axis. As a result, although high performance is obtained, the performance is not generally as high as desirable.